The present invention is directed to communication systems and, more particularly, to a technique for adaptive notch filtering a multiple carrier signal.
As shown in FIG. 1, an exemplary telecommunication system 10 may include mobile units 12, 13A, 13B, 13C, and 13D, a number of base stations, two of which are shown in FIG. 1 at reference numerals 14 and 16, and a switching station 18 to which each of the base stations 14, 16 may be interfaced. The base stations 14, 16 and the switching station 18 may be collectively referred to as network infrastructure.
During operation, the mobile units 12, 13A, 13B, 13C, and 13D exchange voice, data or other information with one of the base stations 14, 16, each of which is connected to a conventional land line telephone network. For example, information, such as voice information, transferred from the mobile unit 12 to one of the base stations 14, 16 is coupled from the base station to the telephone network to thereby connect the mobile unit 12 with a land line telephone so that the land line telephone may receive the voice information. Conversely, information, such as voice information may be transferred from a land line telephone to one of the base stations 14, 16, which in turn transfers the information to the mobile unit 12.
The mobile units 12, 13A, 13B, 13C, and 13D and the base stations 14, 16 may exchange information in either analog or digital format. For the purposes of this description, it is assumed that the mobile unit 12 is a narrowband analog unit and that the mobile units 13A, 13 B, 13C, and 131) are wideband digital units. Additionally, it is assumed that the base station 14 is a narrowband analog base station that communicates with the mobile unit 12 and that the base station 16 is a wideband digital base station that communicates with the mobile units 13A, 13B, 13C, and 13D.
Analog format communication takes place using, for example, narrowband 30 kilohertz (KHz) channels. The advanced mobile phone systems (AMPS) is one example of an analog communication system in which the mobile unit 12 communicates with the base station 14 using narrowband channels. Alternatively, the mobile units 13A, 13B, 13C, and 13D communicate with the base stations 16 using a form of digital communications such as, for example, code-division multiple access (CDMA) or time-division multiple access (TDMA). CDMA digital communication takes place using spread spectrum techniques that broadcast signals having wide bandwidths, such as, for example, 1.2288 megahertz (MHz) bandwidths.
The switching station 18 is generally responsible for coordinating the activities of the base stations 14, 16 to ensure that the mobile units 12, 13A, 13B, 13C, and 13D are constantly in communication with the base station 14, 16 or with some other base stations that are geographically dispersed. For example, the switching station 18 may coordinate communication handoffs of the mobile unit 12 between the base stations 14 and another analog base station as the mobile unit 12 roams between geographical areas that are covered by the two base stations.
One particular problem that may arise in the telecommunication system 10 is when the mobile unit 12 or the base station 14, each of which communicates using narrowband channels, interfere with the ability of the base station 16 to receive and process wideband digital signals from the digital mobile units 13A, 13B, 13C, and 13D. In such a situation, the narrowband signal transmitted from the mobile unit 12 or the base station 14 may interfere with the ability of the base station 16 to properly receive wideband communication signals.
As will be readily appreciated, the base station 16 may receive and process wideband digital signals from more than one of the digital mobile units 13A, 13B, 13C, and 13D. For example, the base station 16 may be adapted to receive and process four CDMA carriers 40A-40D that fall within a multi-carrier CDMA signal 40, as shown in FIG. 2. In such a situation, narrowband signals transmitted from more than one mobile units, such as, the mobile unit 12, may interfere with the ability of the base station 16 to properly receive wideband communication signals on any of the four CDMA carriers 40A-40D. For example, FIG. 3 shows a multi-carrier CDMA signal 42 containing four CDMA carriers 42A, 42B, 42C and 42D adjacent to each other wherein one of the CDMA carriers 42C has a narrowband interferer 46 therein. As shown in FIG. 3, it is quite often the case that the signal strengths of the CDMA carrier signals 42A-42D are not equal.